Intake port structure for internal combustion engines

Abstract

An intake port structure comprises a cylinder head including combustion chambers and supporting an intake valve and an ignition plug in each of the combustion chambers, the intake valve having a stem slidably supported by the cylinder head, the cylinder head having a joint surface to which an intake manifold is connected. A primary intake passage is defined in the cylinder head and communicates with each of the combustion chambers, the primary intake passage having a primary intake port directed toward the stem and the ignition plug, and an opening at the joint surface. The opening has a central axis extending at an angle smaller than 90° with respect to the joint surface. The primary intake passage is smoothly curved between the primary intake port and the opening. The intake manifold has a plurality of primary manifold branches arranged substantially symmetrically with respect to a central axis of the intake manifold.

What is claimed is: 
     
       1. An intake port structure for an internal combustion engine having a plurality of cylinders, comprising: (a) a cylinder head including combustion chambers each having a longitudinal central axis and supporting an intake valve and an ignition plug in each of said combustion chambers, said intake valve having a stem slidably supported by said cylinder head, said cylinder head having a joint surface;   (b) a primary intake passage defined in said cylinder head and communicating with each of said combustion chambers, said primary intake passage having one end defining a primary intake port directed toward said stem and said ignition plug, said primary intake passage at the other end terminating in an opening at said joint surface;   (c) a secondary intake passage defined in said cylinder head and communicating with each of said combustion chambers, said secondary intake passage having a secondary intake port opening into said combustion chamber, and said primary intake port opening into said secondary intake passage;   (d) a two-barrel carburetor having a primary intake passageway for supplying air-fuel mixture to the combustion chambers under a full range of loads and a secondary intake passageway for supplying an air-fuel mixture to the combustion chambers under higher loads, and an intake manifold connected to said joint surface of said cylinder head and to said carburetor, said intake manifold having a plurality of primary branches each of which provides communication from the primary intake passageway to a respective said primary intake passage, said primary branches being arranged symmetrically with respect to a central axis of the intake manifold, said intake manifold also having a plurality of secondary branches each of which provides communication from said secondary intake passageway to a respective said secondary intake passage;   (e) said primary intake passage having a first straight end portion leading to said primary intake port and having a length which is greater than the diameter of said primary intake port, said first straight end portion having a first longitudinal center line which when extended outwardly beyond the primary intake port first intersects the stem of the intake valve and then intersects the periphery of the combustion chamber at a point thereon, said point being connected to the longitudinal center axis of said combustion chamber by a straight line which extends at an angle ranging from 20° to 30° with respect to said first center line when viewed in a plane which extends perpendicular to the longitudinal central axis of said cylinder;   (f) said primary intake passage including a second straight end portion which terminates in said opening at said joint surface, and a smoothly curved arcuate central portion which connects said first and second straight end portions, said arcuate central portion having an arcuate central axis which tangentially contacts said first center line and at a location spaced therefrom also tangentially contacts an imagery straight working line which extends perpendicularly with respect to said joint surface and intersects said first center line at a working point which is adjacent but spaced outwardly from the periphery of said cylinder, said working point being spaced from the longitudinal axis of the intake valve stem by a distance which is greater than the diameter of the secondary intake port, and said arcuate central portion continuing to smoothly curve outwardly beyond its tangential contact with said working line for connection with said second straight end portion;   (g) said second straight end portion having a second longitudinal center line which tangentially contacts said arcuate central axis and which intersectingly extends at an angle of less than 90° with respect to said joint surface when viewed in said plane which is perpendicular to said longitudinal central axis.